This patch is a preparation for @a-binders implementation.  The main changes are:

* Skolemisation is now prepared to deal with @binders.
  See Note [Skolemisation overview] in GHC.Tc.Utils.Unify.
  Most of the action is in
    - Utils.Unify.matchExpectedFunTys
    - Gen.Pat.tcMatchPats
    - Gen.Expr.tcPolyExprCheck
    - Gen.Binds.tcPolyCheck

Some accompanying refactoring:

* I found that funTyConAppTy_maybe was doing a lot of allocation, and
  rejigged userTypeError_maybe to avoid calling it.